In U.S. Pat. Nos. 3,662,411 and 3,684,270, the entire right, title and interest to which has been assigned to the assignee of the present application, there are shown spring assemblies wherein helical coil springs are detachably attached to the wires of a grid frame. In these patents, the grid frame has transversely and longitudinally right-angularly disposed wires which define substantially rectangular seats for the ends of the coils to be attached thereto and the end loops of the coils are provided with segmental portions which extend from the spaces within which the coils are positioned into the next adjacent space beneath the wire defining the spaces and deviations which extend back into the spaces above the wire. The coils can be attached by expanding the end coils to cause the deviations to clear the wires and then allowing the ends to contract to their normal configuration. Alternately, the wires of the grid frame may be displaced toward each other to clear the extremities of the deviations and then allowed to return to their normal spacing. In the aforesaid structures in any given transverse row, the coils are spaced apart the distance of a single space and the transverse rows of coils are spaced longitudinally a distance more or less than a single space. It is desirable sometimes to reinforce, that is, stiffen the assembly in certain areas, notably at the longitudinal edges or in certain localized areas such as the normal positions of shoulders or hips. One way of accomplishing this has been to add coils. However, since the addition of the coils must normally be incorporated at the time of manufacture, specially reinforced spring assemblies must be custom-made. It is one of the purposes of this invention to so construct the assembly of the supporting frame and the grid that the springs can be added to or removed from an already constructed assembly to change the stiffness of the assembly at the edges or within certain interior areas at will without the necessity of using special machinery or tools to thereby avoid the cost of custom manufacture. The base frame of most assemblies is usually a rectangular wooden frame comprising longitudinally-spaced, transversely-extending cross bars to which the lower ends of the spring are attached. As previously explained, the conventional arrangement is to space the coils transversely the distance of a coil apart and to space the transverse rows of coils longitudinally the distance of a coil apart. With such an arrangement, in order to strengthen the assembly, it is necessary to add cross bars for supporting coils between two rows of coils which is objectionable because it adds substantially to the weight of the assembly. It is the purpose of this invention not only to provide for detachably attaching the coils to the grid, but to enable positioning the coils in the spaces between coils in a row without adding cross supports to the base frame and only minor structural changes in the grid.